Glucose homeostasis depends upon the balance between hepatic glucose production and glucose utilization by insulin-dependent tissues, such as fat, muscle and liver, and by insulin-independent tissues such as brain and kidney [Cahill G. F. Jr. (1976), J. Clin. Endocrinol. Metab. 5: 397-415; Bergman R. N. (1989), Diabetes. 38: 1512-1527].
This balance is controlled by pancreatic hormones, insulin from the .beta.-cell of the pancreatic islet and glucagon from the .alpha.-cell. In normal individuals, an increased plasma glucose stimulates insulin secretion. This increase in circulating insulin level promotes glucose utilization by peripheral tissues and inhibits hepatic glucose output.
Non-insulin-dependent diabetes mellitus (NIDDM or Type II diabetes) is characterized by two pathological defects. One defect is insulin resistance of the major target tissues [Himsworth H. and Kerr R. B. (1942), Clin. Sci. 4:120; Kahn C. R. (1978), Metabolism. 27: 1893-1902; Olefsky J. M. (1981), Diabetes. 30:148-161; Reaven G. M. (1988), Diabetes. 37: 1595-1607; Kahn C. R. et al., in Pathogenesis of Non-Insulin Dependent Diabetes Mellitus. Grill V, Efendic S. Eds. (1988) New York Raven p. 227-239; DeFronzo R. A., et al (1992), Diabetes Care 15:318-368; Kolterman G et al. (1981), J. Clin. Invest. 68:957-969]. The other defect is the inability of the pancreas to fully compensate for this insulin resistance [Porte D. Jr. (1991), Diabetes. 40:166-180; Leahy J., et al. (1992), Diabetes Care 15:442-455; Turner R et al. (1992), Ann. Int. Med. 24:511-516]. During the early prediabetic years, insulin secretion is normal or increased. However, insulin secretion finally fails and is unable to compensate for insulin resistance, and it is this relative insulin deficiency that triggers hyperglycemia and clinically manifests Type II diabetes. Both genetic and environmental factors are postulated to be responsible for the progression from normal glucose tolerance to type II diabetes [Defronzo R A, et al (1992), Diabetes Care 15:318-368; Moller D E, Flier J S (1991), N. Engl. J. Med. 325:938-948. Taylor S. I. et al. (1991), J. Clin. Endocrinol. Metab. 73:1152-1163; Kahn C. R., (1994), Diabetes 43:1066-1084]. However, the exact mechanism of the insulin resistance of type II diabetes is still unclear.
Insulin resistance is generally defined as a reduced response to a given concentration of insulin. In Type II diabetes, this is manifested as a decreased ability of insulin to stimulate glucose uptake into muscle and fat, as well as to inhibit glucose production by the liver. In humans with obesity and Type II diabetes, there are multiple defects in insulin action including a decrease in insulin receptor and IRS-1 phosphorylation and a reduced PI 3-kinase activity [Defronzo R. A. et al (1992), Diabetes Care 15: 318-368; Kahn C. R. (1994), Diabetes 43:1066-1084; Kruszynska Y. T., Olefsky J. M. (1996), J. Invest Med. 44: 413-428]. In addition, impaired glucose transporter translocation and stimulation of glycogen synthesis have also been, shown [Rothman D. L. et al. (1992), J. Clin. Invest. 89: 1069-1075; Rothman D. L. et al. (1995), Proc. Natl. Acad. Sci. USA. 92: 983-987; Shulman, G. I. et al. (1990), N. Engl. J. Med. 322: 233-228; Ciaraldi T. P. et al. (1982), Diabetes 31:1016-1022]. Hyperinsulinemia and hyperglycemia, in addition to being secondary manifestations of insulin resistance, also have been shown to induce insulin resistance in target tissues. Insulin resistance in adipocytes is characterized by a decrease in both maximum insulin responsiveness as well as insulin sensitivity of the glucose transport system [Kashiwagi A. et al (1983), J. Clin. Invest. 72: 1246-1254; Marshall S., Olefsky J. M. (1980), J. Clin. Invest. 66: 763-772; Ciaraldi T. P. et al (1982), Diabetes 31: 1016-1022; Kolterman G. et al (1981), J. Clin. Invest. 68: 957-969]. Combined treatment of adipocytes with insulin and glucose causes a rapid and pronounced loss of both maxium insulin responsiveness and insulin sensitivity by impairing the response of translocation of glucose transporters to the cell surface [Garvey W T, et al (1987), J. Biol. Chem. 262: 189-197; Traxinger R R, Marshall S (1989), J. Biol. Chem. 264: 8156-8163].
The hexosamine biosynthesis pathway, in which fructose-6-phosphate is converted to glucosamine-6-phosphate, may be the pathway by which cells sense and respond to ambient glucose levels and, when glucose flux is excessive, down regulate glucose transport resulting in insulin resistant cells [Marshall, S., et al (1991), J. Biol Chem 266:4706-4712]. Glucose induced insulin resistance has been blocked by inhibiting glutamine:fructose-6-P amidotransferase (GFA), the rate-limiting enzyme of the hexosamine pathway [Marshall, S., et al (1991), J. Biol Chem 266:4706-4712]. Glucosamine, an agent known to preferentially enter the hexosamine pathway at a point distal to enzymatic amidation by GFA, bypasses the blockade and is 40-fold more potent than glucose in mediating insulin resistance [Marshall, S., et al (1991), J. Biol Chem 266:4706-4712; reviewed in Marshall S. et al (1991), FASEB J. 5: 3031-3036; McClain D. A., Crook E. D. (1996), Diabetes 45: 1003-1009]. Preexposure to glucosamine induces insulin resistance in skeletal muscle; the tissue responsible for the majority of insulin-dependent glucose utilization. Incubation of rat hemidiaphragm in 5-22 mmol/l glucosamine results in a 20-60% reduction in basal glucose transport and a significant reduction in the ability of insulin to increase glucose transport [Robinson, K. A. et al, (1993), Diabetes 42:1333-1346]. Glucosamine induces insulin resistance in vivo [Baron A. D. et al (1995), J. Clin. Invest. 96: 2792-2801; Rossetti L. et al (1995), J. Clin. Invest. 96:132-140].
A recently implicated important mediator of insulin resistance in obesity and diabetes is tumor necrosis factor-.alpha. (TNF-.alpha.), a cytokine produced primarily by activated macrophages [Beutler B. et al (1985), Nature 316: 552-554] and by adipocytes. TNF-.alpha. is overexpressed in adipose tissues in many animal models of obesity-Type II diabetes [Hotamisligil G. S., Spiegelman B. M. (1994), Diabetes 43: 1271-1278; Hotamisligil G. S., et al (1993), Science 259: 87-91; Skolnik E. Y., Marcusohn J. (1996), Cytokine & Growth Factor Reviews 7: 161-173] and is expressed in increased amounts from the fat of obese insulin-resistant humans [Hotamisligil G. S., et al (1995), J. Clin. Invest. 95: 2409-2415]. It has been shown to downregulate GLUT4 mRNA and protein levels in adipocytes [Hotamisligil G. S., et al (1993), Science 259: 87-91; Stephens J. M. et al (1997), J. Biol. Chem. 272: 971-976]. Administration of TNF-.alpha. to otherwise normal humans or animals results in a reduction in insulin sensitivity [R G. Douglas et al. (1991), Am. J. Physiol. 261, 606-612; T. Van Der Poll et al., ibid., p E457; C. H. Lang et al, Endocrinology 130, 43-52 (1992)]. Neutralization of TNF-.alpha. in obese insulin resistant rats improves insulin receptor signaling and insulin sensitivity of peripheral tissues [Hotamigsil G. S. et al (1993), Science 259: 87-91; Hotamisligil G. S. et al (1994), J. Clin. Invest. 1543-1549]. TNF-.alpha. treatment of cultured 3T3-L1 adipocytes provides a moderate reduction (20-50%) of insulin-stimulated insulin receptor autophosphorylation and a more pronounced effect on IRS-1 phosphorylation [Hotamisligil G. S. et al (1994), Proc. Natl. Acad. Sci. USA 91: 4854-4858; Feinstein R. et al (1993), J. Biol. Chem. 268: 26055-26058]. It has also been suggested that TNF-.alpha. induces insulin resistance via increased serine and threonine phosphorylation of IRS-1 [Hotarnisligil G. S. et al (1996), Science 271: 665-668; Kanety H. et al (1995), J. Biol. Chem. 270: 23780-23784].
Although significant progress has been made in defining the molecular mechanisms of different insulin resistance models, the primary biochemical signaling defects which induce insulin resistance in humans are not known.
Recent data suggest that there may be an association between insulin resistance and oxidative stress. Hyperglycemia and hyperinsulinemia may induce oxidative stress by increased generation of free radicals and reactive oxygen species (ROS) and/or impaired antioxidant defense systems [Wolff S. P., Dean R. T. (1987), Biochem J. 245: 243-250; Kashiwagi A. et al (1994), Diabetologia 37: 264-269; Wohaieb S. A., Godin D. V. (1987), Diabetes 36: 1014-1018]. Hyperglycemia-induced insulin resistance has also been reported to involve at least in part activation of protein kinase C (PKC) [Muller H. K. et al (1991), Diabetes 40: 1440-1448; Berti L. et al (1994), J. Biol. Chem. 269: 3381-3386; Takayama S, et al (1988), J. Biol. Chem 263: 3440-3447]. Further, hyperglycemia induced PKC activation in vascular cells has recently been shown to be prevented by vitamin E [Kunisaki M. et al (1994), Diabetes 43: 1372-1377]. In TNF-.alpha. signaling, increased ROS generation and oxidative stress may play a role. TNF-.alpha. has been shown to stimulate H.sub.2 O.sub.2 production in fibroblasts and chondrocytes [Lo Y. Y. C. et al (1996), J. Biol. Chem. 271: 15703-15707; Sulciner D. J. et al (1996), Mol. Cell Biol. 16: 7115-7121]. ROS have been shown to function as second messengers in TNF-.alpha. induced c-fos expression and antioxidant treatment inhibited the induction of c-fos expression by TNF-.alpha. [Lo Y. Y. C. et al (1995), J. Biol. Chem. 270: 11727-11730; Meier B. et al (1989), Biochem J. 263: 539-545]. Thus, increased oxidative stress and ROS generation may be involved in TNF-.alpha. induced insulin resistance. Oxidative stress may be a common defect in diabetes that links metabolic and obesity-related insulin resistance together.
The current treatment of Type II diabetes includes dietary control, exercise, and stimulation of insulin secretion by oral sulphonylureas. As oral drug therapy aimed at controlling hyperglycemia in NIDDM often fails, insulin therapy is necessary in the late phase of type II diabetes. However, all these approaches do not completely overcome the major defect in type II diabetes: insulin resistance. Therefore, compounds that can correct insulin resistance may be useful in the treatment of NIDDM.